1. Field of the Invention
The present invention relates to a method of driving a liquid crystal display element to display fast moving images.
2. Discussion of the Background
In recent years, liquid crystal display elements have been noted as devices which are thin, light, compact and capable of displaying a large capacity of information, in place of CRTs. As driving methods to such liquid crystal display elements, they are mainly classified into two methods wherein each picture element of a twisted nematic type liquid crystal display element is driven by a thin-film transistor which is disposed in correspondence to each of the picture elements, and a twisted nematic type or a super-twisted nematic type liquid crystal display element is driven without using a thin-film transistor (a simple matrix type).
Although the liquid crystal display element with a thin film transistor can be driven at a relatively high speed, there is a problem that manufacturing steps for preparing the element are complicated and manufacturing cost is high. On the other hand, although manufacturing steps for the simple matrix type liquid crystal display element are relatively simple, there is a problem that it is difficult to switch a display picture at a high speed, whereby it is difficult to obtain a quick response in a display with a mouse at a terminal device when displaying video images.
The reason why it is difficult to drive the simple matrix type liquid crystal display element satisfactorily at a high speed is because the time required for orienting the liquid crystal molecules is large when a voltage is applied to the liquid crystal, which is inherent in the characteristics of the twisted nematic type or the super-twisted nematic type liquid crystal display element. Namely, in such liquid crystal display element having an average response time of about 250 msec, it is impossible to switch a display element or pixel at 20 Hz-30 Hz (which corresponds to a switching time of 33-50 msec) which is generally required in video display.
For high-speed driving, it is considered to use a liquid crystal element having a low response time to a voltage applied to liquid crystal. Such liquid crystal element is called a fast response type liquid crystal element. In order to obtain such fast response type liquid crystal element, there are such a method of using liquid crystal having a low viscosity and such a method that the thickness of the liquid crystal layer is reduced by using liquid crystal having a large refractive index anisotropy.
The response time of the super-twisted nematic type liquid crystal display element is generally in proportion to the viscosity .eta. of the liquid crystal used and is in proportion to the square of the thickness d of the liquid crystal layer used. On the other hand, in consideration of the demand that the product of the refractive index anisotropy .DELTA.n of the super-twisted nematic type liquid crystal display element and the thickness d of the liquid crystal layer should be substantially constant, the response time of the liquid crystal display element is in proportion to the viscosity .eta. and is in inverse proportion to the square of the refractive index anisotropy .DELTA.n. Namely, it is preferable that the thickness d of the liquid crystal layer is small, and liquid crystal having a low viscosity and a large refractive index anisotropy is used for the liquid crystal element.
However, even though a fast response type liquid crystal element can be obtained in a manner as described above, use of such element has encountered an extremely large problem, which is described below. Generally, a method called optimized amplitude selective addressing method (e.g. "LIQUID CRYSTAL TV DISPLAYS" by E. Kaneko, 1987, published by KTK Scientific Publishers) has been used for driving a simple matrix type liquid crystal display element. In the waveform of a voltage applied to line electrodes in the optimized amplitude selective addressing method wherein the number of scanning lines (the number of row electrodes) is N and the frame period is T.sub.F, there is a single selection pulse in the frame period T and a bias wave having a amplitude, which is 1/b as high as an ON voltage selection pulse, in a time other than the application of the selection pulse. Namely, a time of T.sub.F /N is assigned to in a selection time period and a time of (N-1)T.sub.F /N is assigned to a non-selection time period. In FIG. 5a, a symbol A shows a typical waveform of a voltage applied, wherein the abscissa represents time and the ordinate represents voltage. In many cases, two frames are used so as to form an a.c. voltage (d.c. free operation).
In the optimized amplitude selective addressing method, the response characteristic of liquid crystal molecules is effected by the r.m.s. value of the applied voltage to thereby be obtainable a predetermined contrast ratio of display. In FIG. 5b, a symbol C shows a curve of effective value to which the liquid crystal molecules are responsive to the applied voltage wherein the abscissa represents time and the ordinate represents the intensity of transmitting light in a case that polarization plates are arranged at both sides of the liquid crystal layer and an ON voltage is applied to the column electrodes at the time of selection of the line electrodes. Generally, a frame period of about 10 msec--several 10 msec is used, whereas the average response time of a normally used liquid crystal display element is about 250 msec. Accordingly, switching a single pixel of ON or OFF is completed by using several numbers of frames through ten and several numbers of frames.
When a fast response type liquid crystal display element is driven, a change in the direction of the axis of the liquid crystal molecules is apt to follow to the amplitude of voltage applied to the liquid crystal. Accordingly, the transmission of light through the cell as indicated by a wave B in FIG. 5b, the liquid crystal molecules are responsive to peak values, which does not follow the curve C of the integrating responsive characteristic. Namely, there causes a problem that the optical transmission through the cell which rises in a selection time period attenuates in a non-selection time period, whereby the average transmittance level decreases and hence the contrast ratio also decreases. Hereinafter, such phenomenon is called the "relaxation" of liquid crystal.
The relaxation phenomenon causes a serious problem when the number of rows multiplexed (N) is several hundreds or more and a liquid crystal display element having an average response time of about 150 msec or lower is used. In particular, it is considerable when the multiplexing is conducted to a liquid crystal display element having an average response time of about 100 msec or lower.
In this specification, the average response time of a liquid crystal display element is defined as follows.
When a light transmission degree on the application of an OFF voltage at the time when a sufficient time has passed is represented as T.sub.OFF, a light transmission degree on the application of an ON voltage is as T.sub.ON, the time of switching from the OFF voltage to the ON voltage is as t.sub.1, the time when the light transmittance degree T becomes (T.sub.ON -T.sub.OFF).times.0.9+T.sub.OFF after the switching time is t.sub.2, the time of switching from the ON voltage to the OFF voltage again is as t.sub.3, and the time when the light transmission degree T becomes (T.sub.ON -T.sub.OFF).times.0.1+T.sub.OFF after the second switching time is as t.sub.4, the average response time .tau. is expressed as follows: EQU .tau.((t.sub.4 -t.sub.3)+(t.sub.2 -t.sub.1))/2
In order to suppress the relaxation phenomenon, it is considered to utilize a method of increasing the frame frequency to thereby shorten time intervals between selection pulses. In this case, however, a time to select a single line electrode (a pulse width) is necessarily short, and therefore, the reaction of the liquid crystal molecules to the selection pulses is delayed. Accordingly, effect of increasing the contrast ratio of a display is not large.
Further, when the magnitude of a frequency for driving is large, the resistance value of the electrodes is not negligible, so that there causes brightness nonuniformity in a display between a signal inputting portion of an electrode and the other portion, or there causes brightness nonuniformity in a display because of a change of V.sub.th with frequency. For the above-mentioned reasons, it was difficult to use the fast response type liquid crystal display element for the purpose of displaying good images.
On the other hand, T. N. Ruckmongathan proposes a method called Improved Hybrid Addressing Technique wherein a plurality of row electrodes are selected simultaneously or as a batch to drive them (hereinafter, referred to as IHAT method) in order to reduce a driving voltage and to minimize brightness nonuniformity in a display (1988 Internal Display Research Conference). A summary of the driving method is as follows.
An N number of line electrodes are divided into a p number (p=N/M) of subgroups each consisting of an M number of line electrodes, and the M number of line electrodes are selected as a batch to drive them.
A display information of an optional row of column electrodes in a selected subgroup is represented by an M-bit code [d.sub.KM+1, d.sub.KM+2, . . . , d.sub.KM+M ]; d.sub.KM+j =0 or 1 (where 0 designates OFF and 1 designates ON, and k is an integer changeable from 0 to (p-1) in a selected subgroup).
A selection pattern for the line electrodes is expressed by an M-bit code (W.sub.1, W.sub.2, . . . , W.sub.Q) of 2.sup.M (=Q) kinds, i.e., [a.sub.KM+1, a.sub.KM+2, . . . , a.sub.KM+M ]; a.sub.KM+j =0 or 1.
The driving is conducted as follows.
(1) A subgroup is selected as a batch. PA0 (2) An M-bit code is selected as a selection pattern for the line electrodes. PA0 (3) When the line electrodes which are not selected are connected to the ground, the line electrodes selected are applied with -V.sub.r for logic 0 and +V.sub.r for logic 1. PA0 (4) A line electrode pattern for the selected subgroup and a data pattern are compared for each bit by using exclusive logical sum (exclusive OR) to thereby obtain a value of the exclusive logical sum of these data. PA0 (5) A mismatch number i of the two patterns is obtained from the exclusive logical sum. PA0 (6) When the mismatched number is i, a voltage applied to the column electrodes is selected to be V.sub.i. PA0 (7) The voltage applied to the column electrodes is determined independently by repeating the steps (4)-(6) in the matrix. PA0 (8) The voltage is applied to the line electrodes and the column electrodes simultaneously during a time T.sub.R. PA0 (9) A selection pattern is newly selected for the line electrodes, and a voltage applied to the column electrodes is determined through the steps (4)-(6). In the same manner as above, the voltage is applied simultaneously to another line electrodes and column electrodes during a time T.sub.R. PA0 (10) A cycle is completed when a 2.sup.M number of selection patterns are selected once for all subgroups. PA0 (11) A display is refreshed by repeating continuously the cycle. PA0 (1) said j-th row electrode subgroup is selected by applying sequentially voltages so that the elements of a selection voltage vector which constitute a row selection voltage, as defined in the following items (a) and (b), correspond to voltages to the row electrodes constituting the j-th line electrode subgroups: PA0 (2) when said j-th row electrode subgroup is selected under the above-condition (1), the voltage applied to the column electrodes to indicate a display information by means of the vector D.sub.j are determined as in the following items (a) and (b):
In particular, when equations: EQU V.sub.i =V.sub.0 (M-2i)/M, and EQU V.sub.r =V.sub.0 N.sup.1/2 /M
are selected, the ON/OFF ratio of root mean square (r.m.s.) value of voltage can be largest. In this case, the ratio of the root mean square voltage of ON and OFF is expressed by: EQU V.sub.ON /V.sub.OFF =((N.sup.1/2 +1)/(N.sup.1/2 -1)).sup.1/2
The value obtainable is equal to V.sub.ON /V.sub.OFF which is obtainable by using the conventional optimized amplitude selection method. Further, the effective value of voltage at each operating portion in the matrix becomes uniform, whereby a uniform display can be obtained regardless of display patterns.
While the IHAT is effective for reducing the brightness nonuniformity of display, the number of time intervals to complete a cycle is long and hence is not suitable for gray shades using a technique similar to frame modulation. In this case, when the number of row electrodes selected is increased, the number of selection pulses required is sometimes increased as an exponential function. If the width of a selection pulse is uniform, a display requires a time 2.sup.M-1 /M times as much as the conventional method. For instance, if M=7, then 64/7, i.e. a time of 9.1 times is required.